I think it would be awesome for environmental defense tech trees to be implemented as well.

Example: Ice map...you can set traps on frozen lakes which would result in the ice breaking under the army and falling into the ice cold water below, or for avalanches to crush waves of enemies, etc.

Volcanic maps...you'd definitely have to have the top research being one that forced the volcano to prematurely erupt resulting in reduced field of vision, movement, etc. from the ash clouds. You could aptly name it "Pompeii". Just one thing to make the game more unique and vary what could happen in each battle.